I've had the car up on axle stands more or less since it came back from the painters, Its made acsess better for jobs under the car and the working height is a bit better for attaching bumpers and working inside the car.
While spending some *quality time under the car routing a new braided dashboard earth (the one that runs from the roof ariel to the radio and continues all the way down to the chassis.) I couldnt help notice that the exhaust midboxes were still more or less untouched from the Head gasket failiure last year.
At the time I noticed both sides had blown at the point where the input pipe enters the midbox. I made a somewhat half hearted attempt to repair the drivers side with a firegum / aluminum strip kit you get at Halfords... Theory being that I would come back to the issue at some point when the car was on the road, (maybe even using it as an excuse to get a custom stainless system built.)
As it turned out car never really covered any distance and I got distracted by all of the bodywork repairs.
Heres a reminder photo of the blow on the drivers side:
And an identical one on the passenger side:
http://www.industrialbritain.co.uk/foru ... im/871.jpg
Shortly before the car came off the road due to OMGHGF I had an issue with hot running:
Heres the description of the problem from page 18:
Despite the upgrades to the cooling system (alloy radiator and uprated fan) the engine itself seemed to get very hot, Coolant use was minimal and there was no sign of contamination in either the oil or radiator cap. At the start of June under bonnet air temperatures were reaching staggeringly high temperatures the air temperature sensor measured as high as 79ºc while quing in stationary traffic. The car also developed a habit of cutting out when sat at idle; the revs would decline gradually over the course of a 10-15 seconds and would not respond to the throttle if constantly applied. Blipping the throttle would give a response and if you repeatedly blipped the throttle there would generally be a 'pop' which could be heard in the exhaust or the inlet and it would return to idling normally. This problem could also lag IE you could wait at a set of traffic lights and then drive slowly through a village only to have it affect you a few minutes later...
(link http://scimitarweb.co.uk/sgwrs/viewtopi ... 55#p425885
You May also remember these pictures of the carbon buildup on the pistons:
And The strange buildup beneath the inlet valves:
I never entirely found a cause for the HGF and the state of the midboxes nagged at me until I eventaully went online and tried to find a set of replacements. Scimitar midboxes as far as I can tell seem to be unique to the car, my set were stainless, produced by double SS (who made the entire exhaust system.)
Heres a couple of overview shots of the DoubleS system On the car.
I needed an exhaust silencer with input and output of 1" 3/4 bore, the pipe configuration needed to be offset - center in order to get the midbox to line up with the existing system. It also needed to be around 14" in length. Theres acturally a fairly limited amount of choice once you consider all these requirements... Theres more choice at 2" and 2.25" but I did not want the hassle of altering the entire system and I'm not sure a 2" system would be optimal in terms of Exhaust gas velocity.
In the end I found a website which offered a silencer which seemed pretty close:
https://www.304stainlessexhaustparts.co ... c-off.html
A tab with it sat open for several weeks while I waited for payday. a speculative £125 later and I had 2 new silencers....
Heres a side by side comparison new with old...
The current plan is to reuse the input and outlet pipes from the existing midboxes and weld them to the new ones.
I then got a bit keen and cut the old midboxes open....
First glance there seems to be more going on in here than expected, annecdotally GTE Midboxes have a reputation for being a bit restrictive but I had no idea they were this bad. I had expected 2 baffles creating 3 chanbers which gas passed through from front to rear.
Instead this is what was inside:
Ok That last picture is a bit unclear, so heres one with arrow to show the flow of gas through the system...
The gas enters at the bottom right hand side and passes all the way through to the back of the silencer and enters chamber 1, the exhaust then enters the perforated pipe pressurising chamber 2, presumably the pressure and direction change slow the gas and dampen the sound as the gas bounces around this chamber, Gas then gets pushed into chamber 3, From here it exits via the middle pipe. However the weld around the lower (input pipe) is the joint that has failed this suggests that the entire silencer has been under far too much pressure.
I've done a bit of reading online and it seems that good exhaust design is all about achiving the best flow possible while maintaining a scavanging effect and gas velocity. There is a sweet spot that achives a decent balance between all of these factors. Generally any form or directon change or excess pressure in the system is viewed as a major negative.
Its hard to belive that the restriction on the exhaust did not have and affect on the carbon build up in the engine. Whether its the major factor in the HGF failiure is less clear...
I'll finish with a photo looking through the new silencer...
That should be a bit less restrictive. Fingers crossed theres enough clearance for it!